1. Field
The following description relates to an optical imaging system, and more particularly, to a wafer-level lens module with an extended depth of field (EDF) and an imaging device including the wafer-level lens module.
2. Description of the Related Art
Following the development of digital technologies, digital convergence has become increasingly popular. Applications of digital convergence are most active in the field of media and communications. A representative digital convergence product is a so-called “camera phone” in which an imaging device such as a digital camera or a digital camcorder is combined with a mobile phone. Imaging devices such as digital cameras and the like are installed in various mobile electronic devices including laptop computers and Personal Digital Assistants (PDAs) in addition to mobile phones.
As mobile electronic devices with imaging devices need to be small and slimline and are becoming more popular, demands for small, light-weight and low-cost imaging devices are increasing. Particularly, in order to keep pace with the recent trend in which various digital electronic devices, such as MP3 players, portable multimedia players (PMPs), a digital multimedia broadcasting (DMB) televisions, and the like are also being integrated into a camera phone, the demands for small, low-cost imaging devices are further increasing.
One method of manufacturing a small, low-cost imaging device is to reduce the number of elements and to improve the manufacturing process to be more efficient and cost effective. A wafer-level imaging device has been developed to meet this demand. An optical imaging system of such a wafer-level imaging device includes a wafer-level lens module having a stacked structure of substrates, with a lens attached onto one or both sides of each substrate.
Additionally, a camera for a mobile device generally utilizes a voice coil motor (VCM) to vary a focal distance of an optical imaging system to provide auto-focus, macro-focus, and other focusing capabilities. In this case, the VCM is used to adjust the distance between a lens and a sensor so as to change the focal position of the optical imaging system. However, the addition of such a VCM raises manufacturing costs and limits the size reduction of the camera since a space for movement of the lenses has to be ensured.
In order to overcome these limitations, a digital auto-focus technique has been developed to implement auto-focus, and the like using only lenses and an algorithm, without the use of any additional elements such as a VCM. As a digital auto-focus technique, a method of extending the depth of a focus through defocusing has been proposed. An extended depth of field (EDF) lens module can be used for various optical applications, including imaging devices for mobile devices, camcorders, microscopes, security cameras and on the like.
An EDF lens module can form images having somewhat low definition but meeting constant quality criteria over a broad range since it has an EDF. Also, the EDF lens module can recover the definition of the formed images using a digital image processing technique. In such an EDF lens module, since there is a trade-off relationship between a lens design and an algorithm design for image processing, an appropriate balance therebetween is needed. For example, an excellent lens design leads to an algorithm which increases processing speed, but several limitations in lens design make high precision design difficult.
In order to implement the EDF characteristics through an algorithm, a point spread function (PSF) size has to be constant for different wavelengths (e.g. red, green, and blue), for different fields (positions in an image sensor), and for varying distances to an object (infinity, macro, etc.), and the shape of the PSF also has to be uniform. For example, since a short wavelength (for example, a blue ray among visible rays) has a relatively short focal distance, and a long wavelength (for example, a red ray among visible rays) has a relatively long focal distance, both wavelengths do not have enough similarity in spot size, which is a limitation in designing an excellent lens module.
Further, since a wafer-level lens module has a compact size and short distances between lenses, a camera for a mobile device, including the lens module, exhibits picture quality deterioration in a macro-focus mode due to the short focal distance.